1. Field of the Invention
The present invention relates to magnetic materials for magnetic refrigeration, particularly to magnetic refrigeration compositions for magnetic refrigeration suited for use in magnetic refrigerators having a refrigeration initiating temperature of 77.degree. K.
2. Description of the Prior Art
Magnetic refrigeration is a type of refrigeration which is accomplished by the cyclic operations of heat dissipation and heat absorption in the course of magnetization or demagnetization by adding or eliminating outer magnetic fields applied to magnetic substances. It is theoretically the same type of process as that of conventional refrigeration which is accomplished by the cyclic operations of compression and expansion in gas systems.
In low temperature regions, such as below 15.degree. K., the lattice specific heat of magnetic materials for magnetic refrigeration becomes small compared to the magnetic specific heat and a large magnetic entropy change may occur. The lattice specific heat values are negligible so that magnetic refrigeration can be accomplished in low temperature regions by using a magnetic refrigeration cycle of the reverse Carnot type as shown in FIG. 1.
This figure shows the reverse Carnot cycle of A.sub.c .fwdarw.B.sub.c .fwdarw.C.sub.c .fwdarw.D.sub.c on curves of temperature (T) versus total entropy (S/R) of a magnetic substance where S is total entropy and R is the universal gas constant. Total entropy S of a magnetic substance is defined by the equation S=S.sub.L +S.sub.J (T,O)+.DELTA.S.sub.J (T,B), where S.sub.L is lattice entropy, S.sub.J (T,O) is magnetic entropy in zero magnetic field, and .DELTA.S.sub.J (T,B) is magnetic entropy change induced by an external magnetic field. In the present invention, only S and .DELTA.S.sub.J are used, and total entropy is defined as S/R, while magnetic entropy change is defined as .DELTA.S/R. These definitions agree with scientific literature where, in order to facilitate comparison with gas refrigeration, total entropy S and magnetic entropy change .DELTA.S are reported under the convention in which they are divided by the universal gas constant R.
In the A.sub.c .fwdarw.B.sub.c change, a magnetic field is applied to a paramagnetic substance and increases from B=O to B.sub.1 at a temperature of T.sub.1 and thereby causes the paramagnetic substance to undergo isothermal magnetization. Magnetic entropy, that is, entropy of a magnetic moment system, then decreases only by -.DELTA.S.sub.1 /R, so that the magnetic thermal capacity of Q.sub.1 =-.DELTA.S.sub.1 T.sub.1 is released to the outside. In the B.sub.c .fwdarw.C.sub.c change, the outer magnetic field is decreased adiabatically from B.sub.1 to B.sub.2. In this change, the distribution of the magnetic moment occupying each level is substantially unchanged, that is, it is an isoentropic change and the temperature of the magnetic substance is caused to be lowered (adiabatic demagnetization). Thereafter, the reverse Carnot cycle is completed by isothermal demagnetization in the C.sub.c .fwdarw.D.sub.c change and adiabatic magnetization in the D.sub.c .fwdarw.A.sub.c change, which changes are reverse courses of A.sub.c .fwdarw.B.sub.c and B.sub.c .fwdarw.C.sub.c. In the C.sub.c .fwdarw.D.sub.c change, the magnetic substance contrarily absorbs the magnetic thermal capacity of Q.sub.2 =.DELTA.S.sub.2 T.sub.2 and the substance is cooled.
The principle of operation of magnetic refrigeration materials using the above described cycle is explained on the basis of a simplified block figure as shown in FIG. 2. The method of operation of magnetic refrigerators actually manufactured for trial is also introduced in brief as follows:
(i) During isothermal magnetization, heat switch I is closed and heat switch II is opened so that the magnetic field increases up to B.sub.1. The thermal capacity or heating value Q.sub.1 of the magnetic substance released during isothermal magnetization of the magnetic substance is released through the closed heat switch I into a heat source having a high temperature, and the magnetic substance is kept at a temperature of T.sub.1.
(ii) During adiabatic demagnetization, heat switches I and II are opened and the magnetic field decreases from B.sub.1 to B.sub.2. The heat exchange between the magnetic substance and the heat source becomes an isoentropic change because these switches are opened, and the temperature of the magnetic substance is lowered to T.sub.2.
(iii) During isothermal demagnetization, heat switch I is opened and heat switch II is closed so that the magnetic field decreases to zero. Accompanying the decrease of the magnetic field, the magnetic substance absorbs the thermal capacity Q.sub.2 from a heat source having a low temperature (a cooled substance) through heat switch II and increases its self entropy so that cooling is realized.
(iv) During adiabatic magnetization, heat switches I and II are opened and the magnetic field is increased. This is a reverse course of (ii) and the temperature of the magnetic substance rises to T.sub.1 so that it returns to the starting state.
Subsequently, when the cycle is repeated, refrigeration is possible.
Gadolinium gallium-garnet (abbreviated as "GGG" hereinafter) is a paramagnetic material which may be used as a magnetic substance for magnetic refrigerators having refrigeration in low temperature regions, especially in the region below 20.degree. K., using the above reverse Carnot type magnetic refrigeration cycle. In a magnetic substance having a high Debye temperature, such as GGG, the lattice specific heat is almost negligible in the region below 20.degree. K. compared to the magnetic specific heat and a large magnetic entropy change may occur even at a magnetic field of 6 teslas.
However, when a refrigeration initiating temperature is desired at the liquid nitrogen temperature of 77.degree. K., the lattice specific heat of the magnetic substance becomes larger than the magnetic specific heat so that a reverse Carnot type cycle is not usable and an Ericsson cycle must be used. Furthermore, the heat disturbance energy of the magnetic moment also becomes greater so that paramagnetic materials such as GGG described above are not usable.
A reverse Ericsson magnetic refrigeration cycle takes place in the high temperature region above 15.degree. K. or 20.degree. K., namely, in the region of not zero lattice entropy of the magnetic substance and is shown in FIG. 3. This figure shows the reverse Ericsson cycle A.sub.E .fwdarw.B.sub.E .fwdarw.C.sub.E .fwdarw.D.sub.E curves of total entropy (S/R) versus temperatures (T) of the magnetic substance. The difference between this cycle and the reverse Carnot cycle is that the demagnetization or magnetization at isoentropy in the Carnot cycle may be substituted by an isomagnetic field course accompanying entropy changes. In order to obtain a high efficiency approximating that of the Carnot efficiency in a magnetic refrigerator using the reverse Ericsson cycle, magnetic materials are necessary for which the relation of .DELTA.S.sub.1 /R and .DELTA.S.sub.2 /R as shown in FIG. 3 is .DELTA.S.sub.1 /R=.DELTA.S.sub.2 /R. That is, the magnetic entropy change (.DELTA.S/R) removable in the constant magnetic field should be constant in the T.sub.1 to T.sub.2 region, wherein T.sub.1 and T.sub.2 range, for example, from about 20.degree. K. to about 77.degree. K. However, magnetic materials which satisfy the above requirement in such a high temperature region have not been discovered as yet.